1. Field of the Invention
The present invention relates to an image forming apparatus, such as a color printer and a color copier using an electrophotographic recording system, that particularly includes a plurality of image forming units and that can form a color image.
2. Description of the Related Art
In a color image forming apparatus using an electrophotographic system, various color image forming apparatuses using a so-called tandem system are proposed. The color image forming apparatus using the tandem system includes a plurality of image forming units for faster speed and sequentially transfers images of different colors, yellow Y, magenta M, cyan C, and black Bk, to a recording material held on an intermediate transfer belt or on a transport belt. As problems of the color image forming apparatus using the tandem system which includes a plurality of image forming units for faster speed, there are speed fluctuations of a plurality of photosensitive drums or an intermediate transfer belt and meandering of the intermediate transfer belt caused by machine accuracy and other factors.
Therefore, at the transfer position of each image forming unit, an amount of movement of the outer surface of the photosensitive drum and an amount of movement of the intermediate transfer belt vary in each color, and it is difficult to perfectly superimpose the images of each color.
Japanese Patent Application Laid-Open No. 2004-145077 proposes a method of correcting the deviation of the images during image formation, and FIGS. 29A and 29B show the configuration. In FIGS. 29A and 29B, photosensitive members 1M, 1Y, 1C and 1K and an intermediate transfer member 51 include information writing areas that can be rewritten in the sub scanning direction of each surface. The photosensitive members 1M, 1Y, 1C and 1K and the intermediate transfer member 51 include writing units 112M, 112Y, 112C and 112K and 512K that write information in the information writing areas, detection units 113M, 113Y, 113C and 113K and 513M, 513Y and 513C that detect information, and deletion units 111M, 111Y, 111C and 111K and 511 that delete information.
At a transfer section of a photosensitive member 1C, a detection signal of a pattern detected by a detection head 113C and written by a writing head 112C of the photosensitive member 1C and a detection signal of a pattern detected by a detection head 513C and written by a writing head 512K on the intermediate transfer member 51 are compared. As a result, an amount of color deviation based on black can be detected, and the rotational speed of the photosensitive member 1C can be controlled to match the position of the color based on black. The same applies to yellow and magenta.
However, the detection units 113M, 113Y, 113C and 113K and 513M, 513Y and 513C that respectively detect information of the photosensitive drum and the intermediate transfer belt are separately arranged on the front surface and the back surface of the intermediate transfer belt 51 in Japanese Patent Application Laid-Open No. 2004-145077, and the following problem occurs.
A detection unit that detects information written on the photosensitive drum and a writing unit that writes information on the intermediate transfer belt are arranged on the same main scanning line at the image transfer position of the photosensitive drum and the intermediate transfer belt. In this case, a relative positioning error occurs, and a read error and a write error occur when the information on the photosensitive drum is read and written on the intermediate transfer belt.
The positioning error is generally several μm to several dozen μm. Depending on the system, the read error and the write error are generally about several μm due to fluctuations in the tilts of the detection unit and the writing unit or the tilt of the intermediate transfer belt. A relative positional deviation of the detection unit and the writing unit of about several μm may occur due to influence of the temperature.